In a number of digital communications systems, the transmitted signal is formed by linearly adding quadrature-related carrier signals which have been modulated with digital data. Any of a number of specific modulation schemes, known as phase shift keying (PSK), quadrature amplitude modulation (QAM) and amplitude and phase shift keying (APSK), can be used. Such modulation schemes can be presented by a two-dimensional signal-space diagram having four quadrants. Within this diagram, a data point, representing a portion of data from each of the demodulated carriers, is plotted for each of the received data combinations.
In the receiver of these communications systems, the received signal is demodulated using receiver-generated carriers and the digital data is regenerated. These receiver-generated carriers must be synchronous with the carrier signals used at the transmitter. Since the transmitter carrier signals are often suppressed by the modulation process and the propagation through a dispersive transmission channel, the information necessary to phase align the receiver-generated carriers to the transmitter carrier signals must be derived from the received signal. The circuitry which provides these phase aligned, receiver-generated carrier signals is called a carrier recovery circuit.
Carrier recovery circuits can be categorized depending on how the phase aligning or phase error signal is derived. One category includes those circuits which derive a reference signal at the carrier frequency, or some harmonic thereof, by nonlinear processing of the received signal before demodulation. These circuits usually do not meet the low phase jitter objectives required in many system applications. A second category of carrier recovery circuitry, known as baseband carrier recovery circuits, derives a phase error signal by processing the received signal after demodulation. While this second category of circuits reduces the phase jitter, the processing is generally limited to a particular type of demodulation. In addition, many baseband carrier recovery circuits do not provide satisfactory phase error tracking during periods of severe distortion, such as signal fading. It would, therefore, be desirable to develop a carrier recovery circuit which provides low jitter, phase error tracking over a greater range of received signal distortion and is applicable to a number of modulation formats.